636 Mr. D. Sharp and Mr. F. Muir on the Comparative 



or concomitantly with the changes as to the invaginated 

 genital tube. 



Tlie complete invagination of the male structure in 

 the enormous majority of forms is a marked feature of 

 Coleopterous anatomy. Another trait of the Order is 

 the extraordinary extent to which chitinisation is carried. 

 The external parts of Coleoptera are in some cases harder 

 than bone, and in these cases the internal phragmas and 

 apodemes may share in the hardness, as also the male 

 genital tube. For instances we may mention the chitin- 

 isation of this structure in the Histeroid genus Oxysternus, 

 and the Buprestoid Euchroma. A further development 

 of the genital tube is exhibited by elongation, and by 

 chitinisation. We have just mentioned examples of its 

 perfect hardness, and as specimens of its elongation may 

 mention the long flagella so frequently met with, and 

 the remarkable elongation of the sac (or stenazygos) 

 in Eumolpus, where it is about 1| inches long. Turning 

 now to the question of the origin of the sclerites of 

 the tube, we know from the structure of the body wall 

 that exposed large surfaces become very strongly chitinised 

 while immediately contiguous parts remain delicate mem- 

 brane. The chitinisation takes place by the intermediary 

 of hypodermal cells, and it may well be that the reason 

 for parts remaining membranous is due to creases prevent- 

 ing the proper development of hypodermal cells there, and, 

 possibly, their extension in certain directions. 



As the genital tube became elongated it would in the 

 invaginated condition be crumpled and creased, and the 

 formation of separated sclerites on it may probably have 

 been to some extent determined by the nature of these 

 foldings. 



We make these suggestions with a view to getting 

 the student to realise the probability that the develop- 

 ment of the genital tube is due to factors that are on the 

 whole similar to those that have determined the structures 

 of other parts of the body. The factors are not really 

 known. The phenomena of chitinisation are indeed 

 specially obscure, and we are not aware that any one has 

 offered an explanation of the fact that Histers are hard and 

 Malacoderms soft. Neither do we pretend that there is a 

 perfect co-relation between the chitinisation of the sclerites 

 of the body wall and those of the genital tube : in fact we 

 are well aware that in some cases the opposite is true. 



